Process and apparatus for controlling with a computer a distillation column product and heat fluid rates of flow

ABSTRACT

A process and apparatus for controlling a distillation column, especially useful for purifying tolylene diisocyanate. The process comprises (a) selectively proportioning both the reboiler steam flow rate and the column bottoms take-off rate to the distillation column feed rate and (b) adjusting the distillate take-off rate in accordance with variations in the distillation column feed rate and deviations in the level of the column bottoms.

United States Patent Franks et al.

PROCESS AND APPARATUS FOR CONTROLLING WITH A COMPUTER A DISTILLATIONCOLUMN PRODUCT AND HEAT FLUID RATES OF FLOW Inventors: Roger G. E.Franks, Wilmington, Del.;

Steven L. Rltchey, Clark, N].

E. I. du Pont de Nemours and Company, Wilmington, Del.

Filed: July 31, 1970 Appl. No.: 59,941

Assignee:

US. Cl. ..203/l, 203/D1G. 18, 202/206,

202/181, 235/151.l2, 260/453 SP Int. Cl. ..B0ld 3/42, C07c 1 19/04 Fieldof Search..... ..203/] 2, 3, DIG. 18; 202/206,

[56] References Cited UNITED STATES PATENTS 3,143,643 8/1964 Fluegel eta1. ..203/3 3,294,648 12/1966 Lupfer et a1. ....203/2 3,415,720 12/1968Rijnsdorp et a1 ..203/D1G. 18 3,428,528 2/1969 Oglesby et a1 ...203/D1G.18 3,449,215 6/1969 Johnson et a1. ..203/D1G. 18

Primary Examiner-Wilbur L. Bascomb, Jr. Attorney-Melville J. Hayes [5 7]ABSTRACT A process and apparatus for controlling a distillation column,especially useful for purifying tolylene diisocyanate. The processcomprises (a) selectively proportioning both the reboiler steam flowrate and the column bottoms take-off rate to the distillation columnfeed rate and (b) adjusting the distillate take-off rate in accordancewith variations in the distillation column feed rate and deviations inthe level of the column bottoms.

4 Claims, 1 Drawing Figure PATENTEDMAY 21912 3,660,247

E A V 0 3 i it. i 1 CL (,2 8 g z 3 Z 3 INVENTORS I ROGER GABRIEL EUGENEFRANKS STEVEN LEWIS RITCHEY BY g m 3 AGENT PROCESS AND APPARATUS FORCONTROLLING WITH A COMPUTER A DISTILLATION COLUMN PRODUCT AND I'IEATFLUID RATES OF FLOW BACKGROUND OF THE INVENTION This invention relatesto a process and an apparatus for controlling a distillation column tobe used to purify tolylene diisocyanate or other liquids.

Tolylene-diisocyanate is known for its use in the preparation ofpolyurethane foams. One method of preparing tolylene diisocyanate is tophosgenate m-tolylene diamine in a solvent producing a crude reactionmass containing tolylene diisocyanate, solvent and nonvolatiles. Thetolylene diisocyanate is recovered from the crude reaction mass byfractional distillation. The tolylene diisocyanate recovered containsnonvolatile impurities which can range from 20 to 300 parts per million.These impurities are enough'to cause discoloration in polyurethane foamsmade with tolylene diisocyanate. It is, therefore, commerciallydesirable to reduce the amount of impurities from 20 to 300 parts permillion down to a level of about 4 to 100 parts per billion,respectively.

It is known that tolylene diisocyanate can be purified by distillationin a sieve-plate distillation column. The impure tolylene diisocyanateis fed to an intermediate plate in the column. The more volatilematerial, namely tolylene diisocyanate, is obtained and removed from thetop of the still and the nonvolatile or less-volatile materials areobtained and removed from the bottom of the still. If a disturbanceoccurs; for example, if the temperature of the feed stream changes, thetemperature of the cooling water changes, the ambient temperaturechanges, or a flow measurement is inaccurate, the reflux flow rate canchange, thus causing unstable operation of the distillation column. Thereflux serves to wash highboiling impurities down the column where theyare removed in the column bottoms. When the amount of reflux decreasesbelow that needed to obtain the desired distillate purity, the resultingdistillate is commercially unacceptable.

Many methods are known for the control of a distillation column. Apresently used commercial prior art control process includes adjustingthe boil-up rate in proportion to the differential pressure occurringbetween two different selected points in the column. However, corrosionof the numerous small orifices in sieve plates in time can decrease thepressure drop and thus lead to a decrease of the boil-up rate. Inanother presently-used control process, the distillate purity ismaintained by throttling the distillate rate to maintain apre-determined constant temperature of the liquid on a plate close tothe top of the distillation column. This process fails in thepurification desired in tolylene diisocyanate because the concentrationof high-boiling impurities is too small to afiect the tray temperaturecompared to the small fluctuations normally encountered in practice.

Other control processes use computer-techniques to maintain a materialbalance around a distillation column. In the simpler process, pre-setvalues are maintained of the distillateto-feed, reflux-to-feed,boil-up-to-feed, and tails-to-feed flow ratios. Special flow ratecontrol instruments are commercially available for this purpose. In amore complex control process, the composition of the distillate isanalyzed and maintained at a pre-set minimum acceptable value bythrottling the distillate flow rate. In purifying tolylene diisocyanate,the desired purity of the product is so high that it is not practical tomeasure it; thus, it is not practical to base a distillation controlsystem on product analysis. It is also impractical to base a controlsystem on the amount of separation desired because it is impractical tomeasure the amounts being separated.

SUMMARY OF THE INVENTION This invention provides a process and apparatusfor the con- The invention provides a process for controlling adistillation column having a means to feed an impure liquid material tothe column distillate take-off, bottoms take-0E and a steam-heatedreboiler to supply heat to the distillation column, consistingessentially of:

A. measuring and transmitting to an analog computer a signalrepresentative of the rate at which said liquid material is being fed tosaid distillation column,

B. measuring the level of the liquid existing in the bottom of thedistillation column and transmitting a signal representative of thebottoms liquid level to a controller which compares the signal of theactual liquid level to a predetermined desired level and in turntransmits a signal representative of the deviation of the actual bottomsliquid level from the desired level to the analog computer,

C. receiving the signals representing the distillation column feed rateand the deviation of the actual bottoms liquid level from the desiredlevel by the analog computer, automatically proportioning (l) thedesired bottoms take-off rate to the distillation column feed rate bythe equation B xF where B the desired bottoms take-off rate, x apredetermined function and F the distillation columns feed rate, (2) thedesired steam flow rate to the distillation column feed by the equationS yF C,, where S the desired steam flow rate, y a predeterminedfunction, F distillation column feed rate and C is a predeterminedconstant, and (3) the desired distillate take-off rate to thedistillation column feed rate and the deviation of the actual bottomsliquid level from the desired level, by the equation D 2F C F (AL),where D the desired rate of distillate take-off rate, 2 a predetermineda F distillation column feed rate, C is a predetermined constant and ALpercent deviation in bottoms liquid level, and transmitting a signalrepresenting the desired rate of bottoms take-off, the desired steamflow rate, and the desired rate of distillate take-off,

D. measuring and transmitting a signal representative of the actualbottoms take-off rate,

E. controlling the bottoms take-ofi' rate by accepting the signal fromthe computer representative of the desired bottoms take-off rate and thesignal representing the actual bottoms take-off rate, comparing thesignal of the actual bottoms take'off rate to the signal of that desiredand transmitting a signal representative of any deviation to adjust theactual bottoms take-off rate until it conforms to that desired,

F. measuring and transmitting a signal representative of the actualsteam flow rate,

G. controlling the steam flow rate by accepting the signal from thecomputer representing the desired steam flow rate, and the signalrepresenting the actual steam flow rate, comparing the signal of theactual steam flow rate to the signal of that desired, and transmitting asignal representative of any deviation to adjust the actual steam flowrate until it conforms to that desired,

H. measuring and transmitting a signal representative of the actualdistillate take-off rate,

I. controlling the distillate take-ofi rate by accepting the signal fromthe computer representing the desired distillate take-off rate, and thesignal representing the actual distillate take-01f rate, comparing theactual to the desired signal, transmitting a signal representative ofany deviation to adjust the actual distillate take-ofi flow rate untilit conforms with that desired.

This invention also provides the apparatus to carry out the process.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic of a typicaldistillation column incorporating the control system of this invention.

DETAILS OF THE INVENTION The invention will be described as it appliesto a typical tolylene diisocyanate purification. Referring to thedrawing, impure tolylene diisocyanate is fed through conduit 1, into anintermediate region of a sieve-plate distillation column 2. Purifiedtolylene diisocyante vaporizes leaves the distillation column throughconduit 3 and is condensed to a liquid in condenser 4. Liquid tolylenediisocyanate is pumped from condenser 4, through conduit 5, pump 6, andconduit 7, where the stream flow divides directing part of the streamthrough conduit 8 as reflux back to distillation column 2 and theremaining portion through conduit 9 as recovered, purified tolylenediisocyanate.

Heat is supplied to distillation column 2 by continuously feeding andremoving a hot liquid to the bottom of the distillation column. The hotliquid comes from reboiler 10, is fed through conduit 11 intodistillation column 2, and a controlled amount of hot liquid 12 ispermitted to build up in the bottom of the distillation column.Impurities once contained in the vaporized tolylene diisocyanate collectin the hot liquid 12, and the mixture is drawn from the bottom of thedistillation column through conduit 13, pump 14 and conduit 15, wherethe stream divides and part is fed back to reboiler through conduit 16and the remaining portion is directed away from the closed-loop processthrough conduit 17.

The liquid in the reboiler is kept hot by heating it with steam coils.Steam is fed through conduit 18 into steam coils 19 and out throughconduit 20.

Control of this distillation column is maintained by the process of thisinvention wherein the feed rate is set to some desired value and thedistillate take-ofi rate, bottoms take-off rate, and steam flow rate tothe reboiler are proportioned to the feed rate; thereafter, if someperturbation occurs in the distillation process, it has been found thatthe liquid level maintained in the bottom of the still will deviate fromits normal level, and this deviation can be measured and used to bringthe still back into material balance by using the deviation as a signalto operate a flow control valve to control the distillate take-off ratepermitted to leave the closed loop. The amount of flow adjustment madeto the distillate take-off is small compared to the total distillatetake-ofi, but it can be twice as much as the bottoms take-off rate. Thisflow adjustment is thus sufficient to maintain the still in balance whenslight perturbations occur. This adjustment is also important inmaintaining an adequate liquid level in the reboiler so as to keep theheat transfer surface flooded at all times.

The control system is shown in the drawing wherein the amount oftolylene diisocyanate fed to column 2 is sensed by an orifice plate 21and a signal representative of the flow rate is transmitted by flowtransmitter 22 to analog computer 23.

Because an orifice plate and a differential pressure cell are used, thesignal actually transmitted represents the feed rate squared. Thisoccurs because the pressure cell actually measures the difference inpressure across the orifice plate and the feed rate is proportional tothe square root of the pressure measured, i.e., F a therefore, solvingthis equation, APaF. The analog computer will accept the signalrepresenting the feed rate squared and convert this signal to the feedrate itself for use in selectively proportioning the bottoms take-offrate, steam flow rate and distillate take-off rate to the tolylenediisocyanate feed rate signal. (In describing the preferred embodiment,an orifice plate and a differential pressure cell are used to measureall of the actual flow rates; therefore, the flow rate signals actuallyrepresent the flow rate squared. In each instance where the actual flowrate is measured, the receiving apparatus converts the flow rate squaredto the flow rate itself.)

Computer 23 similates three functions which are based on the magnitudeof the tolylene diisocyanate feed signal. First the computer calculatesthe desired rate of bottom flow according to a predetermined andprogrammed equation 24, namely 8 xF where B is the bottoms flow rate,which is equal to a predetermined function x times the tolylenediisocyanate feed rate F. A signal representative of the desired bottomsflow rate is then sent from computer 23 to flow controller 25 where itdetermines the set point for controller 25. The bottoms flow rate issensed by an orifice plate 26, and a signal representative of thebottoms flow rate is sent by flow transmitter 27 to controller 25 whichcompares the actual flow rate to the desired flow rate indicated by theset point and transmits a signal to the bottoms flow rate control valve28 to maintain the bottoms flow rate in accordance with that desired.

The second function performed by computer 23 is to calculate the desiredrate of steam flow to reboiler 10 according to a predetermined andprogrammed equation 29, namely 8 yF C where S (the steam flow rate) isequal to a predetermined function y times the tolylene diisocyanate feedrate F, plus some predetermined constant C,. The constant C is used tomaintain distillation column 2 with a certain minimum reflux rate. Asignal representative of the desired steam flow rate is then sent fromcomputer 23 to steam flow rate controller 30 where it determines the setpoint for controller 30. The steam flow rate is sensed by an orificeplate 31, and a signal representative of the actual steam flow rate issent by flow transmitter 32 to controller 30 which compares the actualsteam flow rate to that desired and transmits a signal to the steam flowrate control valve 33 to maintain the steam flow rate in accordance withthat desired.

The third function performed by computer 23 is to calculate the dfiiredamount of distillate take-off according to a predetermined andprogrammed equation 34, namely, D zF C F(AL), where the desireddistillate take-01f rate, D, is equal to a predetermined function 2times the tolylene diisocyanate feed rate F plus a predeterminedconstant C times the percent bottoms level offset AL times the tolylenediisocyanate feed rate F. The constant C, is used to provide anadjustment of the distillate rate should the distillate rate and bottomsrate be slightly out of balance with the feed rate. If this occurs, itwill show up at once in the bottoms liquid level, and the distillaterate will be adjusted by the equation. In order to make thiscalculation, the computer not only uses the tolylene diisocyanate feedrate, but also receives a signal indicative of the change in the bottomslevel of the distillation column. The amount of liquid used in thebottom of the still is preset to some value sufficient to assuresatisfactory operation of the reboiler. The desired liquid level is usedas the set point; deviations occuring in the liquid level aretransmitted by liquid level transmitter 35 to liquid level controller 36which compares the detected liquid level to the set point and transmitsthe percent liquid level deviation, AL, to computer 23. Computer 23 thencalculates the desired distillation take-off rate by equation 34, namelyD zF C F(AL) and transmits a signal to the distillate flow controller 37where it determines the set point of the controller. The actualdistillate flow rate is sensed by orifice plate 38, and a signalrepresentative of the actual distillate flow rate is sent by flowtransmitter 39 to controller 37, which compares the actual to thedesired distillate flow rate and transmits a signal to the distillateflow control valve 40 to maintain the distillate flow rate in accordancewith that desired.

The functions x, y and z are determined in the following manner:

The material balance around the distillation column can be expressed as:

EQUATION NO. I

F D B where F the rate of feed to the column, D= the rate of distillatetake-off and B the rate of bottoms take-off (all in lbs./hr.). Thematerial balance for the impurities present in the column can beexpressed as:

EQUATION N0. 2

F I,- D], 81,, where FI lbs. impurities in the feed to the the column;D1,, lbs. impurities in the distillate take-off; and B1,, lbs.impurities in the bottoms take-off. The function z is obtained bysolving Equations 1 and 2 simultaneoulsly as follows:

8 F D FI DI (F-D) I F(IF s) D(ID n) r IB)/(ID 8)] Therefore, thedistillate flow rate is proportional to the feed rate by the function z,where Z IF ra/ D n The function x is obtained by solving Equations 1 and2 simultaneously as follows:

D F B FI (F-B) 1,, BI ur D) B(IB n) r ID/IB n) Therefore, the bottomstake-ofi rate is proportional to the feed rate F by the function xwhere:

x I l /I 1,, The function y is derived from the following equation:

where S rate of steam flow, A, heat of vaporization of the isocyanate,C,, the heat capacity of the isocyanate, AT the amount of subcooling ofthe reflux, R the reflux ratio, I the concentration of impurities in theisocyanate feed,* weight fraction) 1,, the concentration of impuritiesin the bottoms take-off, weight fraction) 1,, the concentration ofimpurities in the distillate take-off,* weight fraction) F theisocyanate feed rate, AT the difference between the temperature of theisocyanate feed and the boiling point of the liquid isocyanate on thefeed tray, and A, the heat of vaporization of the steam.

Therefore, the steam flow rate is proportional to the feed rate by thefunction y where [01+ p R) (12+ 1) GEZZM DATF] EXAMPLE As illustrated inthe drawing, a typical fractionating distillation column 2 is used. Heatfor the distillation process is provided by a forced circulationreboiler 10, which is heated with steam. The column is operated at 25Torr overhead pressure and 25 Torr differential pressure. Distillateformed is condensed in a condenser 4.

Tolylene diisocyanate containing about 0.1 weight percent nonvolatilesis fed to about the middle of the distillation column 2 at a rate of10,000 lbs./hr. and a temperature of about 100C. The reflux rate is 4400lbs/hr.

The tolylene diisocyanate feed rate is sensed by an orifice plate 21 andtransmitted to an analog computer 23 by a flow transmitter 22 consistingof a differential pressure cell. The computer calculates the desiredbottoms take-off rate by the formula B xF where x 0.04, F 10,000lbs./hr. and B 400 lbs/hr. A signal representing 400 is sent to abottoms flow rate controller 25 as the set point for the controller. Theset point is compared with the actual bottoms flow rate sensed with anorifice plate 26 and transmitted to the controller by a differentialpressure transmitter 27. The difference between the sipal representingthe actual flow rate and the set point represents the signal sent to abottoms flow control valve 28.

The computer also calculates the desired steam flow rate by the formulaS= yF+ C where y 0.194, C =l 180, and F is the feed rate in pounds perhour (10,000). A signal representing S (3,120) is sent to a steam flowrate controller 30 as the set point for the controller. The set point iscompared with the actual steam flow sensed by an orifice plate 31 andtransmitted to the controller 30 by a differential pressure transmitter32. The difference between the signal representing the actual flow rateand the set point is the signal sent to a steam flow rate control valve33.

The level of the liquid in the bottom of the column 2 is measured by aliquid level detector and transmitter 35 which sends a signalrepresenting the liquid level to a controller 36 which compares thesignal of the actual liquid level to a manually setpoint and thisdifference (AL) is sent to the computer 23. The computer then calculatesthe distillate take-off by the formula D zF+ C (AL)F where z 0.96, C0.000379 (a proportionality constant relating feed rate and columnbottoms level deviation to the distillate rate), F 10,000 lbs./hr., andAL is the variable used to maintain the column in balance and in thisinstance it is 0; therefore, D 9,600 lbs/hr. A signal representing D issent to a distillate take-ofl' rate controller 37 where it is used asthe controller set point.

The actual distillate take-off rate is sensed by an orifice plate 38,and transmitted to the controller 37 by a difierential pressuretransmitter 39. The difference between the signal representing theactual flow rate and the set point is used to adjust a distillatetake-off rate control valve 40 to maintain the distillation column 2 inbalance.

When the feed rate of the tolylene diisocyanate is zero, the computercalculates a zero bottoms flow rate, a zero distillate flow rate, and asteam flow rate of 1,180 lbs./hr. This steam flow will maintain thecolumn at a reflux rate of 4,400 lbs./hr.

When the feed rate is reduced 2,000 lbs./hr. below the 10,000 lbs./hr.,the bottoms rate immediately reduces to 320 lbs./hr., the steam flowrate reduces to 2,370 lbs./hr., and the distillate take-off rateimmediately reduces to 7,680 lbs./hr. The reflux rate is maintained at4,400 lbs/hr.

When the feed rate of tolylene diisocyanate is returned to 10,000lbs./hr., the bottoms flow rate immediately returns to 400 lbs./hr., thesteam flow rate immediately returns to 3,120 lbs./hr., the distillatetake-off rate immediately returns to 9,600 lbs./hr., and the reflux rateremains at 4,400 lbs/hr. This column was operated for 40 continuoushours without producing any defective product.

We claim:

1. A process for controlling a distillation column having a means tofeed an impure liquid material to the column, a steam-heated reboiler tosupply heat to the distillation column, bottoms take-off, distillatetakeoff, and a condenser to condense the distillate consistingessentially of:

A. measuring and transmitting to an analog computer a signalrepresentative of the rate at which said liquid material is being fed tosaid distillation column,

B. measuring the level of the liquid existing in the bottom of thedistillation column and transmitting a signal representative of thebottoms liquid level to a controller which compares the signal of theactual liquid level to a predetermined desired level and in turntransmits a signal representative of the deviation of the actual bottomsliquid level from the desired level to the analog computer,

C. receiving in the analog computer the signals representing thedistillation column feed rate and the deviation of the actual bottomsliquid level from the desired level, automatically proportioning in thecomputer (1) the desired bottoms take-ofi rate to the distillationcolumn feed rate by the equation B xF where B the desired bottomstake-ofi rate, x a predetermined function and F the distillation columnsfeed rate, (2) the desired steam flow rate to the distillation columnfeed by the equation S yF C where S the desired steam flow rate, y apredetermined function, F distillation column feed rate and C is apredetermined constant, and (3) the desired distillate take-E rate tothe distillation column feed rate and the deviation of the actualbottoms liquid level from the desired level, by the equation D 2F C L)F,where D the desired rate of distillate take-off, z a predeterminedfunction, F distillation column feed rate, C is a predetermined constantand L percent deviation in bottoms liquid level, and transmittingsignals representing the desired bottoms take-ofi' rate, the desiredsteam flow rate, and the desired rate of distillate take-off,

D. measuring and transmitting a signal representative of the actualbottoms take-off rate,

E. controlling the bottoms take-off rate by accepting the signal fromthe computer representative of the desired bottoms take-off rate and thesignal representing the actual bottoms take-off rate and transmitting asignal representative of any deviation to adjust the actual bottomstake-off rate until it conforms to that desired,

F. measuring and transmitting a signal representative of the actualsteam flow rate,

G. controlling the steam flow by accepting the signal representing theactual steam flow rate and the signal from the computer representing thedesired steam flow rate, comparing the signal of the actual steam flowrate to the signal of that desired, transmitting a signal representativeof any deviation to adjust the actual steam flow rate until it conformsto that desired,

H. measuring and transmitting a signal representative of the actualdistillate take-off rate,

I. controlling the distillate take-off rate by accepting the signalrepresenting the actual distillate take-off and the signal from thecomputer representing the desired distillate take-off rate, comparingthe actual to the desired signal, transmitting a signal representativeof any deviation to adjust the actual distillate flow rate until itconforms with that desired.

2. The control process of claim 1 in which the liquid being fed to thedistillation column is impure tolylene diisocyanate.

3. The process of claim 2 in which (A) the proportion of the bottomstake-off rate to the distillation column feed rate is in accordance withthe equation: B xF where B the bottoms take-off rate, x 0.04, F feedrate to the distillation column; (B) the proportion of the steam flow tothe distillation column feed is in accordance with the equation: S yF Cwhere S steam flow rate, 0.194, F feed rate to the distillation column,and C 1180; (C) the proportion of the distillate take-off rate to thedistillation column feed rate is in accordance with the equation: D 1FC( L)F where D distillation take-off rate, z 0.96, F feed rate to thedistillation column, L) percent deviation of the bottoms liquid level,and C 0.000379.

4. An apparatus for controlling a distillation column having a means tofeed an impure liquid material to the column, bottoms take-off,distillate take-off, a steam heated reboiler to supply heat to thedistillation column, and a condenser to condense the distillateconsisting essentially of:

A. means for measuring and transmitting to an analog computer a signalrepresentative of the rate at which said liquid material is being fed tosaid distillation column,

B. means for measuring the level of the liquid existing in the bottomsof the distillation column and transmitting a signal representative ofthe bottoms liquid level to a controller which compares the signal ofthe actual liquid level to a predetermined desired level and in turntransmits a signal representative of the deviation of the actual bottomsliquid level from the desired level to the analog computer,

C. means for receiving the signals representing the distillation columnfeed rate and the deviation of the actual bottoms liquid level from thedesired level by the analo computer, automatically proportioning (l) thedesire bottoms take-ofi rate to the distillation column feed rate by theequation 8 xF where B the desired bottoms take-off, X a predeterminedfunction and F the distillation column feed rate, (2) the desired steamflow rate to the distillation column feed rate by the equation S yF Cwhere S the desired steam flow rate, y a predetermined function, Fdistillation column feed rate and C is a predetermined constant, and (3)the desired distillate take-off rate to the distillation column feedrate and the deviation of actual bottoms liquid level from the desiredlevel, by the equation D zF C F L), where D the desired rate ofdistillate take-off, z a predetermined function, F distillation columnfeed rate, C is a predetermined constant and L percent deviation inbottoms liquid level, and transmitting a signal representing the desiredbottoms take-off rate, the desired steam flow rate, and the desireddistillate take-0E rate,

D. means for measuring and transmitting a signal representative of theactual bottoms take-off rate,

E. means for controlling the bottoms take-off rate by accepting thesignal from the computer representative of the desired bottoms take-ofi'rate and the signal representing the actual bottoms take-off rate andtransmitting a signal representative of any deviation to adjust theactual bottoms take-off rate until it conforms to that desired,

F. means for measuring and transmitting a signal representative of theactual steam flow rate,

G. means for controlling the steam flow rate by accepting the signalrepresenting the actual steam flow rate, and the signal from thecomputer representing the desired steam flow rate, comparing the signalof the actual steam flow to the signal of that desired, transmitting asignal representative of any deviation to adjust the actual steam flowrate until it conforms to that desired,

H. means for measuring and transmitting a signal representative of theactual distillate take-off rate, and

I. means for controlling the distillate take-off rate by accepting thesignal representing the actual distillate take-off rate, and the signalfrom the computer representing the desired distillate take-off rate,comparing the actual to the desired signal, transmitting a signalrepresentative of any deviation to adjust the actual distillate flowrate until it conforms with that desired.

t t i i i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3 s ZM-Y Dated May 2, 'I 972 inventofls) Roger G. E. Franks andSteven L. Ritchey It is certified that error appears in-theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

IN THE SPECIFICATION Col. 2, line 3).;., after predetermined, "a" shouldread function 001. 3, line 59 "AP F" should read ---AP -F Col. 5, line23, B (IF ID/IB ID) (F)" should read IN THE- CLAIMS Claim. 1 deleteparagraph G. and insert: 7

C. receiving in the analog computer the signals representing thedistillation column feed rate and the deviation of the actual bottomsliquid level from the desired level, automatically proportioning in thecomputer (i the desired bottoms take-off rate to the distillation columnfeed rate by the equation B xF where B the desired bottoms takeoff rate,1 x a predetermined function and F the distillation columns feed rate,(2) the desired steam flow rate to the distillation column feed by theequation S yF 0 where S' the desired steam flow rate, y a predeterminedfunction, F distillation column feed rate and C is a predeterminedconstant, and (3) the desired distillate take-off rate to thedistillation column feed rate and the deviation of the actual bottomsliquid level from the desired level, by the equation D 2F C (AIJF, whereD the desired rate of distillate take-off, z a predetermined function, Fdistillation column feed rate, C

is a predetermined constant and A L percent deviation in bottoms liquidlevel, and transmitting signals representing the desired bottomstake-off rate, the desired steam flow rate, and the desired rate ofdistillate take-off,

FORM PO-105O (IO-69) USCOMM'DC 60376-P69 V US, GOVERNMENT PRINTINGOFFICE I969 0-366-334,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRETION Patent No.3,660,247 Dated May 2 1 972 Roger G. E. Franks and Steven L. RitcheyInventor(s) Page 2 I It is certifieddthat error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Claim 3, delete in its entirety and insert:

3, The process of claim 2 in-which (A) the proportion of the bottomstake-off rate to the distillation column feed rate is in accordance.with the equation! B xF where B the bottoms take-off rate, x 0.0L F feedrate to the distillation column; (B) the proportion of the steam flow tothe distillation column feed is in accordance with the equation: S yF Cwhere S steam flow rate, y 0.1 9h, F feed rate to the distillationcolumn, and C1 11 80; (C) the proportion of the distillate take-off rateto the distillation column feed rate is in accordance with the equation:D zF C (A L)F where D distillation take-off rate, 2 0.96, F feed rate tothe distillation column, (A L) percent deviation of the bottoms liquidlevel, and C2 0.000379. 0

Claim L delete paragraph 0. and insert:

C. means for receiving the signals representing the distillation columnfeed rate and the deviation of the actual bottoms liquid level from thedesired levelv by the analog computer, automatically proportioning (1the desired bottoms take-off rate to the distillation column feed rateby the equation B xF where B the desired bottoms take-off, x apredetermined function and F the distillation column feed rate, (2) thedesired steam flow rate to the distillation column feed rate by theequation S yF C he e S the desired steam flow rate, y a predeterminedfunction,

F distillation column feed rate and C is a predetermined constant, and(3) the desired distillate take-off rate to the distillation column feedrate and the deviation of actual bottoms liquid level from the desiredlevel, by the equation D zF C F(A L), where D the desired rate ofdistillate FORM PO-105O (10- 9) USCOMM-DC 60376-P69 I Q U45. GOVERNMENTPRINTING OFFICE: I955 0-366-334 v UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3,660,214] Dated May 2, 972

Roger" G. E. Franks and Steven L. Ritchey Page 3 Inventor(s) It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

' take-off, z a predetermined function, F distillation column feed rate,C is a predetermined constant and A L percent deviation in bottomsliquid level, and transmitting a signal representing the desired bottomstake-off rate,

the desired steam flow rate and the desired distillate take-off rate,

Signed and sealed this lSth'dayof August 1972.

(SEAL) Attest':

EDWARD M.FLETCHER,JR. 7 ROBERT GOTTSCHALK Attesting Officer ICommissioner of Patents FORM PO-105O (10-69) USCOMM-DC 60376-P69 usGOVERNMENT PRINTING OFFICE: I969 0-366-384

2. The control process of claim 1 in which the liquid being fed to thedistillation column is impure tolylene diisocyanate.
 3. The process ofclaim 2 in which (A) the proportion of the bottoms take-off rate to thedistillation column feed rate is in accordance with the equation: B xFwhere B the bottoms take-off rate, x 0.04, F feed rate to thedistillation column; (B) the proportion of the steam flow to thedistillation column feed is in accordance with the equation: S yF + C1,where S steam flow rate, y 0.194, F feed rate to the distillationcolumn, and C1 1180; (C) the proportion of the distillate take-off rateto the distillation column feed rate is in accordance with the equation:D zF + C2( Delta L)F where D distillation take-off rate, z 0.96, F feedrate to the distillation column, ( Delta L) percent deviation of thebottoms liquid level, and C2 0.000379.
 4. An apparatus for controlling adistillation column having a means to feed an impure liquid material tothe column, bottoms take-off, distillate take-off, a steam heatedreboiler to supply heat to the distillation column, and a condenser tocondense the distillate consisting essentially of: A. means formeasuring and transmitting to an analog computer a signal representativeof the rate at which said liquid material is being fed to saiddistillation column, B. means for measuring the level of the liquidexisting in the bottoms of the distillation column and transmitting asignal representative of the bottoms liquid level to a controller whichcompares the signal of the actual liquid level to a predetermineddesired level and in turn transmits a signal representative of thedeviation of the actual bottoms liquid level from the desired level tothe analog computer, C. means for receiving the signals representing thedistillation column feed rate and the deviation of the actual bottomsliquid level from the desired level by the analog computer,automatically proportioning (1) the desired bottoms take-off rate to thedistillation column feed rate by the equation B xF where B the desiredbottoms take-off, X a predetermined function and F the distillationcolumn feed rate, (2) the desired steam flow rate to the distillationcolumn feed rate by the equation S yF + C1, where S the desired steamflow rate, y a predetermined function, F distillation column feed rateand C1 is a predetermined constant, and (3) the desired distillatetake-off rate to the distillation column feed rate and the deviation ofactual bottoms liquid level from the desired level, by the equation DzF + C2 F( Delta L), where D the desired rate of distillate take-off, za predetermined function, F disTillation column feed rate, C2 is apredetermined constant and Delta L percent deviation in bottoms liquidlevel, and transmitting a signal representing the desired bottomstake-off rate, the desired steam flow rate, and the desired distillatetake-off rate, D. means for measuring and transmitting a signalrepresentative of the actual bottoms take-off rate, E. means forcontrolling the bottoms take-off rate by accepting the signal from thecomputer representative of the desired bottoms take-off rate and thesignal representing the actual bottoms take-off rate and transmitting asignal representative of any deviation to adjust the actual bottomstake-off rate until it conforms to that desired, F. means for measuringand transmitting a signal representative of the actual steam flow rate,G. means for controlling the steam flow rate by accepting the signalrepresenting the actual steam flow rate, and the signal from thecomputer representing the desired steam flow rate, comparing the signalof the actual steam flow to the signal of that desired, transmitting asignal representative of any deviation to adjust the actual steam flowrate until it conforms to that desired, H. means for measuring andtransmitting a signal representative of the actual distillate take-offrate, and I. means for controlling the distillate take-off rate byaccepting the signal representing the actual distillate take-off rate,and the signal from the computer representing the desired distillatetake-off rate, comparing the actual to the desired signal, transmittinga signal representative of any deviation to adjust the actual distillateflow rate until it conforms with that desired.